The present invention relates to an illegal copy prevention apparatus of preventing written property such as computer software or the circuit configurations of semiconductor IC devices, from being copied illegally.
Recently, various illegal copy prevention apparatuses have been proposed to prevent computer software or circuit configurations of semiconductor IC devices from illegal copying. For instance, the same inventors have already proposed a prevention apparatus in U.S. Pat. application No. 269,358, now abandoned as disclosed below:
In case programs are illegally copied from an original floppy disk to another floppy disk and then the illegally copied floppy disk is driven to run programs, illegal copy discriminating means discriminates that the floppy disk is illegal. On the other hand, an event generating means generates a constant event, at a predetermined probability. Therefore, when the illegal copy detection and the event generation both occur simultaneously, an abnormal operation generating means generates an abnormal operation to disable the operation.
As described above, conventionally, the abnormal operation has been generated at a relatively low probability without generating an abnormal operation whenever illegal copying has been detected. This is because a violator who illegally copies software, for instance, cannot easily know whether the software can be copied normally or not. Therefore, after the illegally copied floppy disks have been put on the market and then used, an abnormal operation will occur in some floppy disks, so that the violator who illegally copied software notices a serious situation for the first time. Consequently, illegal copying has been prevented by not allowing the violator to remove the copy protection. That is, it has been impossible for the violator who illegally copies software to sell the illegal floppy disks, because it is difficult to discriminate whether the copied software is identical to the original one or if the protection can be removed completely.
In the above-mentioned prior-art illegal copy prevention apparatus, however, there still exists a problem, as follows. Although illegal copying can be detected by the illegal copy discriminating means and further an abnormal operation can be generated only when the event generating means generates an event at a predetermined probability, when the probability at which an event is generated is high, the violator who copies software illegally can easily discriminate whether software can be copied normally or not, with a result that the violator can compare the illegally copied software with the original one repeatedly for perfect analysis, until an abnormal operation will not be generated and therefore the copy protection function can be removed perfectly. In contrast with this, when the probability at which an event is generated is determined low, the copy protection may not be removed because the violator can not notice the presence of the copy protection. In this case however, since almost all the illegally copied software can endure to use, there exists another problem in that it is impossible to effectively prevent illegal copying.
The above-mentioned problems can be applied to circuit configuration of semiconductor IC devices by the same reason as in program software. Therefore, it has been a serious problem how to generate an abnormal operation whenever circuit configuration to be protected is copied illegally.